Yahoo employees collectively forfeited that right when they "cashed out" of their holdings, and sold their stakes to outsiders in sufficient quantities to let those outsiders control their destiny.
As a normal non-employee shareholder of Yahoo, I am pissed off at the value destruction Yahoo has done, so it is well within my rights to demand that they accept this too-good-to-pass-up offer.
They cannot have their cake and eat it too - they wanted the liquidity and the exit, and now they cannot whine.
(Yes, most current employees never participated in the bonanza, but they did join a company where former employees had the bonanza and then jumped ship - who asked them to join such a diluted company, if they wanted to be in control of their own destiny?)
Same applies to start-ups here: if you sell yourself, don't later claim that the new owner did this or that. If you want to control your destiny, stay private or at least majority private. Of course that contradicts the quest for exit that motivates so many here, ...
With capitalism, it's all about doing what's best for the company. It's not about the individual at all.
Don't get me wrong: capitalism is great -- provided that you are the CEO.